In a recent usenet posting in reply to a query about where to obtain Acorn Mice, Ian Nichols commented that he'd had success modifying standard optical PS/2 mice for use on RiscPCs.

As you may know, Acorn machines from the Archimedes up to the RiscPC have an atypical mouse interface, normally referred to as "quadrature". Outside of Acorn machines, these were also widely used on Sun machines and terminals. Historically, most mice on other platforms were serial, and later PS/2 and more recently, USB. Many mice now available for purchase work on both PS/2 and USB interfaces. PS/2 is also used for the keyboard interface on RiscPCs.

Even at the time of the RiscPC manufacture, the quadrature interface was unfashionable, and the story goes that Acorn only stuck with it because of supplies of components. They later ditched this, and A7000s and A7000+s received a PS/2 interface, as was the norm on PCs at the time.

Still, this left existing users who required a new mouse a relatively expensive replacement option from Acorn. To the rescue came STD, who created a PS/2 adaptor allowing attachment of the growing range of PS/2 mousing options from the likes of Logitech. Later, STD produced a smaller and cheaper option, the PS2MouseMini, a small cable adapter performing much the same job. Soon after, Castle produced a similar product. Both STD and Castle also produced their own Acorn-compatible mice.

DIYing it
In November 2003, Ian Nichols tells his readers, he discovered that some optical mice (those are the ones without wheels, but use LED reflections to determine motion) contain chips that are able to output quadrature signals as well as the PS/2 signalling. Screwdriver and soldering iron in hand, and using a cable from an Acorn mouse, Ian went onto to modify several mice for himself and fellow club members.

If you fancy having a go yourself, and saving a few pennies, then Ian has recently documented the whole thing, on his website.